The present invention relates to magnetic data storage and retrieval systems and, more particularly, to such systems in which a magnetoresistive sensor is used in the retrieval of magnetically stored data.
Magnetic data storage systems are used to store data in a moving magnetic media layer provided on a moving disk through use of an electrical current-to-magnetic field transducer, or "write head", positioned immediately adjacent thereto. The data is stored, or "written", to the magnetic media by switching the direction of flow of a substantially constant magnitude write current which is established in conductive windings in the write head. Each write current direction transition results in a reversal of the magnetization direction in that portion of the magnetic media just passing by the write head during the flow established in the new direction with respect to the magnetization direction in the media induced by the previous flow in the opposite direction.
Magnetic data retrieval systems are used to recover data previously written to a magnetic media through use of a magnetic field-to-electrical voltage transducer, or "read head". The read head is positioned to have the magnetic media containing previously-written data pass closely thereby such that flux reversal regions in that media create time-varying magnetic fields that can be sensed to provide corresponding output signals. Magnetoresistive (MR) sensors are advantageously used by the read head for this purpose since the resistivity of an MR sensor positioned near the magnetic media fluctuates in response to magnetic fields emanating from the rotating magnetic disk. These changes in resistivity of the MR sensor are easily converted into a usable output voltage signal by establishing a current through the MR sensor.
Typically, a differential amplifier is used as a bias monitoring amplifier for monitoring bias voltage across the MR sensor and temperature. First and second input-follower circuits are often used as buffers between the MR sensor and the differential amplifier to limit the effects of the impedance of one on the other. In bipolar transistor based follower circuits (i.e., emitter-follower circuits), the first and second emitter-follower circuits generally each include both a bipolar transistor and a means for supplying current through that transistor. The bipolar transistors in the first and second emitter-follower circuits each generally have a base connected to a corresponding one of the first and second sides of the MR sensor and a collector connected to a source of voltage. The transistor emitters are connected to a corresponding one of the first and second inputs of the differential amplifier and through a corresponding load impedance to another source of voltage.
Although first and second emitter-follower circuits work well in buffering the MR sensor from the differential amplifier, they each have a corresponding small, but not zero, output impedance that results in a differential error in the output voltage of the differential amplifier when first and second input currents into respective first and second inputs of the differential amplifier are unequal. The unequal first and second input currents correspond to unequal output voltages at the outputs of the respective first and second emitter-follower circuits, thereby causing the voltage difference error between first and second inputs of the differential amplifier.
In some prior art applications, a feedback loop around the emitter-follower circuits has been used to equalize the emitter-follower transistor collector currents, and to correspondingly equalize the amplifier first and second input currents. Such a feedback loop monitors the currents through the first and second emitter-follower circuits as indicators of the circuit output voltages, and provides voltage indications of those currents to the inputs of a differential amplifier. A current that depends upon the voltage difference at the inputs to the differential amplifier is provided at the output of the appropriate one of the first and second emitter-follower circuits to effectively cause the first amplifier input current to substantially equal the second amplifier input current. Although such feedback loops can be effective, they require a relatively large number of circuit components (and so a large amount of die area) to implement and a relatively large amount of power to operate. There is, therefore, a need for read sensor buffers and a signal amplifier combination requiring fewer circuit components than prior art buffers and amplifiers to equalize the first and the second amplifier input currents.